1. Field of the Invention
The present invention relates to a semiconductor storage device and a method of operating the same, and in particular, to a semiconductor storage device with ferroelectric capacitors and a method of operating the same.
2. Description of the Related Art
Ferroelectric memory (FeRAM) is a semiconductor storage device that allows non-volatile storage of binary data based on two different magnitudes of polarization in a ferroelectric substance, using hysteresis characteristics of ferroelectric capacitors. Memory cells in a conventional ferroelectric memory include ferroelectric capacitors and transistors that are connected to each other (see, for example, Japanese Patent Laid-Open No. 2001-250376). The ferroelectric memory (FeRAM) includes a memory cell array having a plurality of memory cells arranged therein.
In the ferroelectric memory, a selection transistor, which is provided at an end of a memory cell array including a memory cell to be read, is selected and made conductive to connect the memory cell to a bit line. Then, plate voltage is applied to a plate line connected to the memory cell, and voltage is applied to each end of a ferroelectric capacitor included in the memory cell. Electric charges from the ferroelectric capacitor are read to the bit line, while a reference potential is applied to the other of a pair of bit lines (the complementary bit line) by a reference potential generation circuit. Then, the potentials of the pair of bit lines are compared and amplified by a sense amplifier. Thus, the difference between the amounts of electric charges read to the pair of bit lines represents the amount of signals.
The amount of signals is affected by the position of a memory cell array in the chip, and the parasitic capacitance of a word line, a bit line, and so on connected to the memory cell. In the ferroelectric memory, such impacts on the amount of signals read to the pair of bit lines are problematic because of reduction in the read operation margin.